A new twist in the photophysics of the GFP chromophore: a volume-conserving molecular torsion couple.

Jamie Conyard,Ismael A Heisler,Philip C Bulman Page,Lluís Blancafort,Yohan Chan,Stephen R Meech

doi:10.1039/c7sc04091a

Abstract

The simple structure of the chromophore of the green fluorescent protein (GFP), a phenol and an imidazolone ring linked by a methyne bridge, supports an exceptionally diverse range of excited state phenomena. Here we describe experimentally and theoretically the photochemistry of a novel sterically crowded nonplanar derivative of the GFP chromophore. It undergoes an excited state isomerization reaction accompanied by an exceptionally fast (sub 100 fs) excited state decay. The decay dynamics are essentially independent of solvent polarity and viscosity. Excited state structural dynamics are probed by high level quantum chemical calculations revealing that the fast decay is due to a conical intersection characterized by a twist of the rings and pyramidalization of the methyne bridge carbon. The intersection can be accessed without a barrier from the pre-twisted Franck-Condon structure, and the lack of viscosity dependence is due to the fact that the rings twist in the same direction, giving rise to a volume-conserving decay coordinate. Moreover, the rotation of the phenyl, methyl and imidazolone groups is coupled in the sterically crowded structure, with the methyl group translating the rotation of one ring to the next. As a consequence, the excited state dynamics can be viewed as a torsional couple, where the absorbed photon energy leads to conversion of the out-of-plane orientation from one ring to the other in a volume conserving fashion. A similar modification of the range of methyne dyes may provide a new family of devices for molecular machines, specifically torsional couples.

Highlights

In the following we investigate the photophysics of a novel sterically crowded variant of the chromophore of the green uorescent protein (GFP), by means of ultrafast spectroscopy and high-level quantum chemical calculations
In contrast to the native green fluorescent protein (GFP) chromophore, whose decay is calculated to be governed by ring rotation, the presently calculated excited state structural evolution suggests that the methylated derivative follows a barrierless, volume conserving coordinate composed of ring rotation and pyramidalization of the central carbon
A number of GFP chromophores have been previously synthesized with a view to stabilizing the planar geometry and enhancing uorescence

Summary

Introduction

Examples include the primary step in the vision pigment rhodopsin, bacterial phototaxis stimulated by the photoactive yellow protein and uorescent protein (FP) photochromism used in super-resolution microscopy.[1,2,3] These efficient protein based photoswitches inspired the design of diverse molecular photoswitches, which power a variety of nano- and micro-scale phenomena.[4,5,6,7,8,9] In the following we investigate the photophysics of a novel sterically crowded variant of the chromophore of the green uorescent protein (GFP), by means of ultrafast spectroscopy and high-level quantum chemical calculations This bridge methylated derivative (see Scheme 1) shows an exceptionally fast excited state decay which is almost independent of solvent viscosity. We further show that this arises from a nonplanar form of the chromophore analogous to that found in some photoactivated FPs.[10]

Methods

Results

Conclusion